Bd. VI: 4) THE ECHINOIDEA. 47 



1 1 mm. diameter (Stat. 59) the height is larger than half the diameter. But upon 

 the whole it must be said that the height of the test is distinctly smaller than in 

 St. diadema, the species to which it is nearest related, and from which it is not 

 distinguished without some difficulty (comp. measurements of the latter species given 

 in »Echinoiden der deutschen Siidpolar-Expeditiom p. 75). 



The whole shape of the test differs from that of diadema, besides by the greater 

 flatness, in being regularly round in outline, whereas in diadema it is somewhat 

 pentagonal. The test is not flat on the abactinal side (as in antarcticus), but generally 

 distinctly conical, somewhat more so than in diadema, in which latter it is some- 

 what more arched. — The edge of the actinostome is distinctly sunken, as pointed 

 out by DODERLEIN, whereas it is almost flat in diadema (PI. VI. Figs. 10 — 12). 



The number of plates in the ambulacra and interambulacra is upon the whole 

 as in diadema, as far as can be concluded from the comparatively small material of 

 the latter species. There is, however, some variation in the number of plates; thus 

 a specimen of 58 mm. diameter has 33 — 34 ambulacral and 18 interambulacral plates, 

 whereas a specimen of 56 mm. diameter has 38 — 39 ambulacral and 19 interambu- 

 lacral plates. As a general rule the number of ambulacral plates is about double 

 of that of the interambulacral plates; only in the youngest specimen, 11 mm. dia- 

 meter, there are only 17 — 18 ambulacral plates against 11 interambulacral. — The 

 relative width of the areas is mainly the same in the two species; in the larger 

 specimens the interambulacra are, at the ambitus, about twice as broad as the am- 

 bulacra, in younger specimens they are somewhat narrower. 



To DODERLEIN's description of the tuberculation I have to add that the scro- 

 bicular areas in the interambulacra are in contact at and below the ambitus, almost 

 to the peristome (PI. VI Figs. 10, 12); in one specimen (Stat. 16, 60 mm. in diameter, 

 otherwise quite a typical specimen) they are, however, not in contact, being sepa- 

 rated even at the ambitus by a distinct series of miliary tubercles. In the ambulacra 

 the scrobicular areas may join for a short space at the ambitus in the larger speci- 

 mens. DODERLEIN states that the small tubercles, though partly larger on the actinal 

 side, never form distinct series; this is also the case in some of the larger speci- 

 mens before me (the smaller specimens must, of course, be left out of consideration 

 here), but in most of the specimens the secondary tubercles form, in the inter- 

 ambulacra, distinct longitudinal series, both inside and outside the primary series. 

 This is in so far a good distinguishing character, the secondary tubercles not forming 

 longitudinal series on the actinal side in diadema; but, it must be conceded, it is 

 no constant character of Agassizii. The small secondary tubercles at the ambitus 

 may be more or less distinctly arranged in transverse series. 



The apical system and the anal area are of the same size as in diadema, and 

 the character of the anal area is the same: a small, but mostly distinct, central plate 



